Raman spectroscopy is a proven and effective technique for qualitatively or quantitatively detecting specific chemical compounds within a liquid medium. Monochromatic probe light of a known frequency or wavelength is directed into the liquid. A portion of the probe light photons momentarily change the energy states of individual molecules within the liquid. The excited molecules then emit Raman signal photons which have frequencies that differ from that of the probe light by amounts which characterize and identify the particular molecular species. Thus the presence of one or more molecular species can be detected and quantified if desired by analyzing the scattered probe light for the characteristic Raman signals of such species.
Raman signals can be very weak making detection difficult. Surface enhanced Raman spectroscopy is a known technique for increasing Raman signal emission. In particular, it has been found that constituents of a liquid which are adsorbed on the surface of certain metals, such as silver, gold or platinum, exhibit greatly increased Raman signal emission by factors of up to 10.sup.6 in some cases.
Prior apparatus for practicing suface enhanced Raman spectroscopy has tended to be undesirably complex, bulky and costly and is also subject to operational limitations. Under some conditions, the Raman signals of interest are diluted or masked by fluorescence originating in portions of the liquid that are away from the metal adsorbtion surface. Conventionalapparatus does not localize the analysis to Raman emissions from molecules adsorbed on the metal surface at least to a desirable extent. In investigating reactions in an electrochemical cell, for example, it may be desirable to obtain signals only from the immediate region of an electrode where initial reduction or oxidation reactions occur. The composition of the cell fluid away from the electrode may be distinctly different due to subsequent reverse reactions.
The prior role of optical fibers in surface enhanced Raman spectroscopy has been the limited one of transmitting probe light into the liquid medium and/or returning scattered light for frequency analysis. probe light is directed into one end of the fiber and the other end is spaced from the metal surface in position to direct the probe light to substances adsorbed on the surface and/or to receive light which has been scattered by such substances. This does not resolve the above discussed deficiencies of the conventional apparatus at least to the desirable extent.
The present invention is directed to overcoming one or more of the problems discussed above.